Field of the Invention
The invention relates to the field of optical metrology.
More particularly, it relates to a spectrometer of high diffraction efficiency for analysing the spectrum of a light beam or a light source.
The present invention finds a particularly advantageous application when a high efficiency of the spectrometer over a broad spectrum band is desired.
Description of the Related Art
In optical metrology, spectroscopy is a technique that consists in analysing the spectrum of an upstream light beam, whether the latter comes directly from a light source or from an object illuminated by a light source, and in deducing therefrom certain properties of this source or this object.
A spectrometer is an optical instrument allowing to perform such an analysis for a spectrum comprising a plurality of wavelengths.
It is well known that a spectrometer generally includes:                an entrance slit letting the upstream light beam through,        collimation means generating, from the upstream light beam, a collimated light beam,        angular dispersion means intercepting the collimated light beam and angularly dispersing the collimated light beam according to a plurality of wavelengths,        detection means adapted to measure light intensities according to the plurality of wavelengths.        
In many applications, as for example Raman spectroscopy or near-infrared spectroscopy, the quantity of light available in the spectrometer, at the detection means, for the spectrum analysis is low. Indeed, a spectrometer carries all the photons of the upstream light beam from the entrance slit to the detection means, with many losses, due in particular to the angular dispersion means.
Moreover, even if the angular dispersion means may be optimised so as to reduce the losses in such a spectrometer, this can be done only on a narrow spectrum band.
Hence, a fast or accurate measurement with such a spectrometer may prove difficult, except using high-performance but expensive detection means.
From documents US2010/0225856A1 and US2010/0225876A1 are known, for example, angular dispersion means comprising so-called achromatic, polarization-separation diffraction gratings, which have a very high diffraction efficiency in the diffraction orders +1 and −1 over a broad spectrum band, in particular in the domain of ultraviolet, visible and infrared wavelengths.
On the other hand, such achromatic polarization-separation diffraction gratings able to be used in a spectrometer are known from the article C. Oh and M. J. Escuti, “Achromatic polarization gratings as highly efficient thin-film polarizing beamsplitters for broadband light”, Proceedings of SPIE, vol. 6682, no. 668211 (2007).
However, by construction, a polarization-separation diffraction grating operates as a polarization separator. Therefore, the measurement performances of a spectrometer using such a component are not uniform as a function of the polarization state of the upstream light beam, the light intensities measured by the detection means varying with the fluctuations of the polarization state of the upstream light beam.